Fuel cells electrochemically convert fuels and oxidants to electricity, and fuel cells can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). Some artisans consider the acronym "PEM" to represent "Polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
Usually, an individual PEM-type fuel cell has multiple, generally transversely extending layers assembled in a longitudinal direction. In the typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. As is known in the art, some of the fluid manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates for each fuel cell. Also, other fluid manifolds circulate coolant (e.g., water) for cooling.
As is known in the art, the PEM can work more effectively if it is wet. Conversely, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely. So, the fuel and oxidant fed to each fuel cell are usually humidified. Furthermore, a cooling mechanism is commonly employed for removal of heat generated during operation of the fuel cells.
Flow field plates are commonly produced by any of a variety of processes. One plate construction technique, which may be referred to as "monolithic" style, compresses carbon powder into a coherent mass. Next, the coherent mass is subjected to high temperature processes which bind the carbon particles together, and convert a portion of the mass into graphite for improved electrical conductivity. Then, the mass is cut into slices, which are formed into the flow field plates. Usually, each flow field plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions. Typically, flow field channels are engraved or milled into a face of the rigid, resin-impregnated graphite plate. Undesirably, permeability of the graphite and machining processes therefor limit reduction of plate thickness. So, one is disadvantageously limited from increasing the number of corresponding fuel cells which occupy a particular volume in a fuel cell stack, and which can contribute to overall power (voltage, current) generation. Moreover, resin-impregnated graphite plates are susceptible to brittle failure and expensive in terms of cost of raw materials, as well as time for processing and tool wear in machining.
Another known flow field configuration places a mattress of metal-wire fibers between a bipolar plate and an electrocatalytic electrode, which is in turn adjacent to an ion exchange membrane. The mattress of metal-wire fibers acts as distributor for the reactants and products, in addition to providing deformability and resiliency in the electrochemical cell. The bipolar plate can omit flow channels and is formed from aluminum or other metal alloys. Such a design is disclosed in U.S. Pat. No. 5,482,792 to Faita et al. (entitled "Electrochemical Cell Provided With Ion Exchange Membranes and Bipolar Metal Plates," issued Jan. 9, 1996, and assigned to De Nora Parmelec S.p.A.) and U.S. Pat. No. 5,565,072 to Faita et al. (entitled "Electrochemical Cell Provided With Ion Exchange Membranes and Bipolar Metal Plates," issued Oct. 15, 1996, and assigned to De Nora Parmelec S.p.A.). A shortcoming of this design is the material resource expense and weight in constructing the bipolar plate entirely from metal. A further shortcoming is the material resource expense and weight, as well as the space consumption, in providing the mattress of metal-wire fibers. In addition, the formation of the geometric features for the metal plate is expensive in terms of time and tool wear in machining.
Thus, a need exists for a flow field plate, and a fuel cell assembly of which it is a part, allowing formation thereof with decreased use of conductive material, which material is expensive and heavy. A further need exists for such a flow field plate, and fuel cell assembly, in which flow channel(s) and other geometric feature(s) can be shaped to optimize fluid(s) service, such as reactant fluid flow to the membrane and cooling therefor. Also, a need exists for such flow channel(s) and other geometric feature(s) to be easily formable.